Problem: Mark has $\frac{3}{4}$ of a dollar and Carolyn has $\frac{3}{10}$ of a dollar. How many dollars do they have altogether? (Give your answer as a decimal.)
Explanation: Mark has $\frac{3}{4}$ of a dollar, or $75$ cents.

Carolyn has $\frac{3}{10}$ of a dollar, or $30$ cents.

Together, they have $75+30=105$ cents, or $\boxed{\$1.05}.$